The present invention relates to a gravity base structure for an offshore platform and, more particularly, to a structure of this type designed to be erected in waters infested with icebergs and to withstand subsequent collision with the latter.
Barbaras et al U.S. Pat. No. 4,906,136 filed by the present assignee, discloses a platform structure of this type which comprises a concrete monolith caisson supported by a bottom slab resting on the seabed, this caisson having a configuration of vertical teeth which form defensive elements capable of withstanding the impacts of icebergs and of absorbing the energy transmitted. The caisson also comprising a top slab directly or indirectly supporting the deck of the platform and, between the two slabs, a peripheral double wall which is formed by two concentric walls interconnected by vertical partitions which form a lattice structure of triangular prisms, the outer concentric wall carrying the defensive elements.
The defensive elements having the shape of teeth break up the surface of the iceberg when it impacts, and dissipate the energy of the impact and transmit the forces resulting from the impact to the structural elements of the peripheral double wall.
A platform of this type well withstands iceberg impacts. However, economic considerations have induced a critical study of this platform with the aim of reducing its construction cost while at the same time preserving, or even improving, the resistance of the platform to the impact of icebergs.
In this respect, it has been noted that the defensive elements of the platform of the above mentioned patent consist of reinforced concrete walls presenting a larger outer surface area with an important quantity of concrete and reinforcements. With reference to FIGS. 2 and 4 of the abovementioned patent, it furthermore appears that the use of defensive elements attached to the external face of the outer wall of the caisson implies a presence of partitions of the double wall leading to six-branched "nodes" which are difficult, and therefore expansive, to produce.
It will also be noted that the defensive elements with a triangular shape according to the embodiment in FIG. 4 of the above-mentioned patent are inclined quite steeply to the surface of the outer wall. Such an arrangement creates substantial tangential forces within the structure resulting from an oblique iceberg impact on the caisson. As a consequence, it is then necessary to reinforce the structure against such forces, which has a further unfavorable effect on its cost.